The present invention relates generally to pressure sensing probes and more particularly to a non-invasive pressure sensing method and apparatus for measuring internal pressure of a vessel or cavity having at least a portion of the one wall made up of an elastic membrane or in which an elastic membrane can be fitted.
The use of non-invasive measurement has received considerable attention in recent years. In particular, in the fields of physiology and medicine, this method has seen considerable development and utilization in cardiology, obstetrics and ophthalmology.
While the use of non-invasive pressure measuring devices, or tonometers, for applications such as measuring ocular pressure to determine the existence of glaucoma and measuring intracranial pressure of newborns, is well known, the existing methods and apparatus utilized are inaccurate and, for some applications, impractical.
One known method of non-invasive tonometer predominantly used in medical applications contemplated for the present invention utilizes a single pressure transducer and coplanar "guard ring" developed in the late 1950's. Such a device is discussed in Mackay, R.S. and Marg E., "Fast Automatic Ocular Pressure Measurement based on exact theory". IRE Trans. Med. Electronics, ME-7, 61-67, (1960) and Smyth, C.N. "The Guard-Ring Tocodynamometer: Absolute Measurement of Intra-amniotic Pressure by a New Instrument," J. Obstetrics and Gynecology, 64, 59-66 (1957).
As depicted in FIG. 1 hereto, the prior art guard ring tonometer, 1, consists of a single central force transducer, 3, surrounded by an annular ring, 5. As shown in FIG. 2, the force transducer, 3, is disposed within the annular ring 5 and is coplanar therewith.
In operation, the guard ring tonometer, 1, is advanced against the surface of a pressurized elastic membrane of the vessel under review, as shown in FIGS. 3-5, causing the pressure sensed by the central transducer, 3, to increase as the tonometer, 1, is advanced and the membrane of the pressurized vessel, contacting the tonometer, 1, is increasingly flattened.
When the membrane just covers the central transducer, 3, [FIG. 4] the sensed pressure is at a maximum value, since the internal pressure of the vessel and the pressure required to bend the membrane will both be measured by the transducer, 3.
As the tonometer, 1, is further advanced toward the vessel, [FIG. 5] the bending force component of the measured pressure is transferred outwardly to the fixed annular ring, 5, causing the measured pressure at the central pressure transducer, 3, to reach a minimum. At this point, the pressure registered by the central transducer, 3, is equal to the pressure internal to the vessel.
In essence, the fixed coplanar annular ring, 5, acts as a "guard ring" to prevent the bending forces from affecting the measurement. As the tonometer, 1, is further advanced, the indicated pressure again increases owing to the increase in pressure caused by the tonometer displacement or invagination of the pressurized membrane.
An alternative way of describing this technique is to note that upon depression of the guard ring, 5, sufficiently to applanate the elastic membrane of the vessel, the radius of the membrane apparent to transducer, 3, becomes essentially infinite. Thus, there is no contribution in the measured pressure due to bending forces, because the effect of any bending force components occurs outside the radius detected by the transducer, 3.
The use of the "guard ring" tonometer, 1, [and all current related methods and devices] as described above, has several difficulties associated with its operation, especially when used for long term, continuous measurements.
For example, it is difficult to determine when to stop advancing the tonometer, even for short, single sample measurements. One may use a readout device to sense the increase, decrease and increase in pressure as described in the use of this device. But it will be difficult to know just when to stop advancing the tonometer. Moreover, if pressure is variable, either due to the vessel itself or because of application force or movement of the tonometer, rapid and accurate adjustments to the tonometer to ensure accurate readings by manual methods based on the increase and decrease of pressure are difficult at best. Also, if the tonometer is attached to the membrane by utilizing a fixturing method, the tonometer may move or come loose after it was properly mounted or the pressure of the vessel may change.
Unless the operator continually moves the tonometer about to determine if it is at the point of minimal pressure, it is thus unclear whether or not the membrane remains properly applanated for accurate pressure measurements. The "guard ring" tonometer method of non-invasive pressure measurement is therefore effective, if at all, for a single measurement sample, taken over a brief period of time.
One attempt to solve this problem, discussed in the Marg reference cited above, was to provide transducers at a given periphery in the guard ring and to measure the pressure upon simultaneous activation of these sensors. This did not, however, ensure against unequal pressure being applied to some of the periphery transducers, thereby resulting in improper pressure readings. This method also fails to provide for anything but single discrete measurements, because movement of the tonometer may continue to activate the peripheral sensors while creating a false increase in the pressure reading due to added application pressure on the guard ring tonometer.
Another known, and more recently developed, method of non-invasive pressure measurement, utilized for intracranial pressure applications, inVolves a transducer that is slowly forced against the subject membrane by a lead screw. Such a device is described in Majors et al., "Intracranial Pressures Measured with the Coplanar Transducer". Med. Biol. Eng. 10, 724-733 (November 1972). As With the "guard ring" tonometer approach, the accuracy of this method is dependent upon operator skill and experience in observing various pressure readings as the transducer is applied against the subject vessel and selecting the optimum application force for sensing the pressure. This approach also fails to address problems associated with changing conditions invalidating original settings discussed above with reference to the guard ring tonometer.
Many attempts at measuring intracranial and ocular pressure and similar applications have been made by using single pressure transducers, with and without guard rings. In all cases, the difficult problem remains as to when to stop the application of the transducer against the membrane to make a pressure reading that one can call the actual vessel pressure. A related and serious problem is that if one has indeed found the correct application pressure and mounts a transducer against the membrane to make a reading, changing conditions of internal pressure in the vessel or of the means of mounting the transducer to the membrane will invalidate the readings of pressure when the transducer either looses its coplanarity or is actually being applied with too much pressure, thereby recreating bending forces on the membrane and thus increasing the vessel pressure. One survey of just such attempts involved in the measurement of intracranial pressure and confirmation of these problems associated with the simple application of a single transducer to a membrane can be found in, "Clinics in Perinatology/Noninvasive Neonatal Diagnosis," February 1985, edited by Alistair G.S. Philip, M.D., published by W.B. Saunders Co.
No general provision exists in the known non-invasive pressure sensing methods and devices for simple precise control of the application force to ensure accurate pressure readings. While the known sensing devices provide for instantaneous, discrete measurements, the accuracy of these measurements and the possibility of continued monitoring of the tonometer is subject to operator skill and experience, as well as the changes and pressure of the vessel under investigation and movement of the tonometer itself. It is, therefore, of substantial interest for medical and similar applications involving the need to measure the pressure of a vessel provided with a flexible, external membrane to provide a method and apparatus to simplify and improve the precision of pressure measurement on both an instantaneous and continual basis, and that this method and apparatus be adapted for ease of adjustment where changes and conditions related to the vessel pressure and application of the pressure transducer(s) vary.